If you feel that LibreOffice is misbehaving by having its own menu, then you can force it to integrate into the system. There's a special package for just that. Install, re-login into your desktop session and enjoy. ...
Indeed, to resolve this, install the LibreOffice menubar package:
sudo apt-get install lo-menubar

Click to expand...

I tried it quite sometime ago and am not happy. For one, the "global menu" doesn't have "Window" as one of the menu options. There's a "technical" reason cited for that and some sort of workaround. But the main issue is that keyboard shortcuts are kaput: Alt+F won't drop down the File menu. Alt+E won't give you the Edit drop-down and so on. I don't know if these things are fixed. ... I suspect we may have to wait for the Pangolin

In any case, I don't have strong feelings one way or the other about the global menu. Keyboard shortcuts suffice.